MySQL table Partition with FLOOR function (partition function not allowed)?

MySQL documents the partitioning functions here. The floor() function appears to have some special considerations. In this case, I think the issue might be that the division is returning a float/double result rather than a decimal result.

This is easily fixed in your case because you do not need to do the division:

CREATE TABLE `fact_events` (
    `event_key` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
    `event_type_key` tinyint(3) unsigned NOT NULL,
    `analytic_file_id` bigint(20) unsigned NOT NULL,
    `sdk_session_id` bigint(20) unsigned NOT NULL,
    `virtual_button_create_id` bigint(20) unsigned NOT NULL,
     PRIMARY KEY (`event_key`),
     KEY `idx_events_event_type` (`event_type_key`)
 ) PARTITION BY RANGE(event_key) (
     PARTITION p0 VALUES LESS THAN (0),
     PARTITION p1 VALUES LESS THAN (100000),
     PARTITION p2 VALUES LESS THAN (200000),
     PARTITION p3 VALUES LESS THAN (300000),
     PARTITION p4 VALUES LESS THAN (40000000),
     PARTITION p5 VALUES LESS THAN MAXVALUE
 )

EDIT:

As an explanation. floor() returns an integer value but not necessarily an integer type. Here is the documentation:

For exact-value numeric arguments, the return value has an exact-value numeric type. For string or floating-point arguments, the return value has a floating-point type.

When you divide two integers, you produce a floating point argument and this is not appropriate for partitioning.